Self-frequency doubled Nd-doped ycob laser

ABSTRACT

Neodymium-doped yttrium calcium oxyborate (Nd:YCOB) is the single active gain element for a solid-state laser device capable of achieving both lasing and self-frequency doubling optical effects. A pumping source for optically pumping Nd:YCOB can generate a laser light output of approximately 400 mW at approximately 1060 nm wavelength and a self-frequency doubled output of approximately 60 mW at approximately 530 nm wavelength. Thus, a laser device can be designed that is compact, less expensive and a high-powered source of visible, green laser light.

[0001] This invention relates to solid-state laser devices, and inparticular to a new type of compact, high-power laser with frequencydoubling capabilities to generate coherent visible light, and claimspriority to U.S. Provisional Patent Application S. N. 60/118,301, filedFeb. 2, 1999.

BACKGROUND AND PRIOR ART

[0002] The laser is a device for the generation of coherent, nearlysingle-wavelength and single-frequency, highly directionalelectromagnetic radiation emitted somewhere in the range fromsubmillimeter through ultraviolet and x-ray wavelengths. The word laseris an acronym for the most significant feature of laser action: lightamplification by stimulated emission of radiation.

[0003] There are many different kinds of laser, but they all share acrucial element: each contains material capable of amplifying radiation.This material is called the gain medium, because radiation gains energypassing through it. The physical principle responsible for thisamplification is called stimulated emission. It was widely recognizedthat the laser would represent a scientific and technological step ofthe greatest magnitude, even before T. H. Maiman constructed the firstone in 1960. Laser construction generally requires three components forits operation: (1) an active gain medium with energy levels that can beselectively populated; (2) a pumping process to produce populationinversion between some of these energy levels; and usually (3) aresonant electromagnetic cavity structure containing the active gainmedium, which serves to store the emitted radiation and provide feedbackto maintain the coherence of the electromagnetic field.

[0004] In a continuously operating laser, coherent radiation will buildup in the cavity to a level set by the decrease in inversion requiredbalancing the stimulated emission process with the cavity and mediumlosses. The system is then said to be lasing, and radiation is emittedin a direction defined by the cavity.

[0005] A common approach to converting the laser wavelength to half itsvalue (for example, from 1064 nm to 532 nm), often used to convertinfra-red lasers to lasers emitting in the visible part of the spectrum,is to use intra-cavity frequency up conversion (IC). The most common ICapproach is to incorporate a second crystal, a non-linear opticalcrystal, correctly oriented for phase matching, inside the laserresonator, and to adjust the reflectivity of the cavity mirrors tomaximize the wavelength converted laser light emission.

[0006] The lasers of the present invention use a new crystal material asthe active gain medium. The new gain medium is trivalent neodymium-dopedyttrium calcium oxyborate referred to herein as Nd³⁺:YCa₄O(BO₃)₃ orNd:YCOB for easier reference. Patent Corporation Treaty (PCT)application numbered WO 96/26464 reports the growth of calciumgadolinium oxyborate, GDCOB, as the first element of a new family ofborate crystals. The abstract for WO 96/26464 states, “The crystals areprepared by crystallising a congruent melting, composition of generalformula: M₄LnO(BO₃)₃, wherein M is Ca or Ca partially substituted by Sror Ba, and Ln is a ianthanide from the group which includes Y, Gd, Laand Lu. Said crystals are useful as frequency doublers and mixers, as anoptical parametric oscillator or, when partially substituted by Nd³⁺, asa frequency doubling laser.” Although, the general formula might beinterpreted to include various Nd-doped crystals, the PCT application,WO 96/26464, only demonstrates and claims Nd-doped GdCOB or LaCOB.Additionally, the subject inventors have discovered that the orientationof axes and angles for the demonstrated crystals disclosed in WO96/26464 are not efficient for a self-frequency doubling laser. Moreimportantly, WO 96/26464 does not demonstrate nor claim any method norapparatus for using Nd-doped YCOB as a self-frequency doubling laser.

[0007] In the prior art, there are no disclosures of Nd:YCOB as anactive gain medium or as the gain medium in a harmonic generation laser.As a member of the oxyborate family of crystals, the non-hygroscopicYCOB crystal possesses nonlinear optical properties and when doped withNd³⁺ ions, the new crystals have the advantage of combining the activegain medium and the nonlinear frequency conversion medium in a singleelement. Self-frequency doubled (SFD) lasers are an attractivealternative to conventional intra-cavity frequency doubling with aseparate nonlinear crystal such as potassium titanyl phosphate (KTP), asdisclosed in U.S. Pat. No. 4,942,582. A SFD laser incorporates lowerreflection, absorption and scattering losses and leads to a simpler andmore robust resonator design. With the addition of diode-pumping, theNd:YCOB laser provides a new type of compact, high-powered, visiblelaser light source.

[0008] Trivalent neodymium-doped crystalline laser systems producingoptical radiation are reported. U.S. Pat. No. 4,942,582 supra, disclosesa single frequency solid state laser having an active gain medium whichcomprises a block of neodymium doped crystals of vanadium oxide (YVO₄),garnet (YSGG) and borate (YAB) in combination with a separate frequencydoubling crystal of KTP (potassium titanyl phosphate, or KTiOPO₄); thisinvention overlooked the self-frequency doubling possibilities of theNd:YAB crystal. U.S. Pat. No. 5,058,118 disclosed that a single crystalof neodymium doped borate (Nd:YAB) was useful as a self-frequencydoubling minilaser generating a 0.532 μm (green light) and 0.660 μm (redlight) laser beam. However, this laser configuration suffers from pooroptical quality and self-absorption at 530 nm as disclosed in J. Appl.Phys., Vol. 66, pp. 6052-6058, 1989.

[0009] More recently, the approach to generating high power, visiblelaser light has been to use nonlinear optical crystals to convertnear-infrared radiation to the visible portion of the spectrum viasecond harmonic generation (SHG) (sometimes termed frequency doublingand used interchangeably, herein). This process generates a harmonicwavelength that doubles the frequency of the fundamental wavelength.Since the SHG conversion efficiency is a function of the fundamentallaser beam intensity, the nonlinear crystal is often placed inside thecavity of a low-power continuous wave laser to benefit from the higherintracavity fundamental beam intensity. This technique is discussed inU.S. Pat. No. 5,610,934 and U.S. Pat. No. 5,751,751 which provides anexample of frequency doubling when neodymium doped crystals of vanadiumoxide (YVO₄) or (GdVO₄) are bonded to non-linear crystals of potassiumniobate (KNbO₃) or β barium borate (BBO). A fundamental beam of about914 nm is frequency doubled to produce blue laser light at about 457nanometers (nm) or (0.457 μm).

[0010] U.S. Pat. No. 5,802,086 discloses a continuous wave (cw)microlaser with a highly absorbing solid-state gain material, preferablyneodymium-doped yttrium orthovanadate (Nd:YVO₄) that lases at twofundamental wavelengths and are frequency-mixed in a nonlinear crystaloriented within the cavity to generate a third wavelength which maybedifficult to generate directly or by frequency doubling.

[0011] Popular host crystals including garnet, especially yttriumaluminum garnet (YAG) and yttrium orthovanadate (YVO4) are discussed inthe prior art. However, the search for smaller, less expensive, morepowerful, multifunctional lasers continues. The discovery of a new classof laser hosts, the oxyborates, makes possible the combination of linearand nonlinear optical properties in a single active medium. Moreparticularly, the neodymium-doped oxyborate crystal (Nd:YCOB) of thepresent invention generates a near infrared fundamental light which canbe self-frequency doubled to provide a compact, efficient, source ofvisible green laser light.

SUMMARY OF THE INVENTION

[0012] The first objective of this invention is to use Nd:YCOB as anactive gain medium for a laser.

[0013] The second objective of this invention is to provide aself-frequency doubled (SFD) laser using the oxyborate familyof-crystals as the host crystal.

[0014] The third objective of the present invention is to provide avisible light laser that combines the active gain medium and frequencydoubler in one single element.

[0015] The fourth objective of this invention is to provide a compactefficient source of visible laser light.

[0016] A preferred embodiment of the invention provides aneodymium-doped oxyborate crystal (Nd:YCOB) pumped with either tunableor continuous wave (cw) coherent, diode pumped, or titanium:Sapphirelaser radiation or near infrared light in a range from approximately 760nanometer (nm) to approximately 840 nm, with highest absorption atapproximately 792 nm and approximately 812 nm. The preferred embodimentefficiently generates 530 nm of green laser light.

[0017] The optical pumping means which provides energy to the crystalcan be selected from one of a coherent or incoherent light pumpingsource. The incoherent pumping source may be xenon or krypton lamps orlight emitting diodes (LED) or laser diodes, which can be of pulsed orcontinuous wave output. The coherent pumping source may be a laser lightsource, such as a single laser diode or a matrix laser diode series,which can also be of pulsed or continuous wave output.

[0018] Further objects and advantages of this invention will be apparentfrom the following detailed description of a presently preferredembodiment, which is illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

[0019]FIG. 1 is an experimental laser cavity design. F. I., Faradayisolator; HWP, half-wave plate; lens, 8.8-cm PL/CX lens; HR, 5-mROCmirror; X-tal, 2% Nd:YCOB; OC, 10-cm ROC output coupler.

[0020]FIG. 2 is Fundamental output power at wavelength vs. absorbedTi:Sapphire pump power for 2% Nd:YCOB.

[0021]FIG. 3 is Self frequency-doubled (SFD) output power vs. absorbedpump power with 5% Nd:YCOB active gain medium.—3 a) Ti:Sapphire pumped;3 b) Diode pumped.

[0022]FIG. 4 shows Orientation of X, Y, Z optical indicatrix axesrelative to the crystallographic axes and planes of Nd:YCOB. The typicalboule cross-section is also indicated.

[0023]FIG. 5 is crystal orientation for optimum self-frequency doubling(SFD) laser action.

[0024]FIG. 6 is the absorption spectrum for 5% Nd:YCOB for lightpolarized parallel to the X, Y, Z axes shown in FIG. 5 supra.

[0025]FIG. 7 is Emission spectrum for 5% Nd:YCOB as a function ofpolarization relative to X, Y and Z axes.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] Before explaining the disclosed embodiments of the presentinvention in detail it is to be understood that the invention is notlimited in its application to the details of the particular arrangementshown since the invention is capable of other embodiments. Also, theterminology used herein is for the purpose of description and not oflimitation.

[0027] The Czochralski method, as reported by Qing Ye and Bruce H. T.Chai in the Journal of Crystal Growth 197 (1999) 228-235; “Crystalgrowth of YCa₄O(BO₃)₃ and its orientation” is used to grow the oxyboratecrystal for the present invention. When rare-earth elements, such as,neodymium are added during the crystal growing process, the crystal issaid to be “doped” with the rare-earth ions. Doping changes the functionof the crystal into an active gain medium, since undoped YCOB crystal isa nonlinear optical medium, doping of the crystal with an impurityconverts the crystal into a nonlinear optical laser crystal.

[0028] When the newly formed crystal is grown from the melt, it isgenerally in a cylindrical shape called a crystal “boule.” The boule canbe cut into a cylindrical rod or other geometric shapes. To make it intoa laser crystal it generally requires two flat ends. The flat ends arepolished and given an appropriate reflective coating or anti-reflectivecoating. One end is more reflective than the other; laser light isemitted through the end that is less reflective—the output coupler.

[0029] Undoped YCOB has been shown to have a nonlinear coefficient,d_(eff) of 1.1 pm/V, which is between that of other nonlinear crystalsKDP (0.37) and BBO (1.94 pm/V). See J. Appl. Phys. 36, 276 (1997) and W.Koechner, Solid State Laser Engineering, 4 th ed. (Springer-Verlag NewYork, 1996), p. 579.

[0030] A concentration of Nd exceeding 5% or more of the doping changesthe refractive indices of the crystal; which in turn, changes theoptimum angle for phase matching. Nd doping for the present invention ispreferably in a range from approximately 2 to approximately 20 weight %of the YCOB crystal, with optimum doping at approximately 10%.

[0031] Initial experiments were performed to investigate the potentialof Nd:YCOB as a laser medium. A simple hemispherical laser system pumpedby a tunable cw Ti:sapphire laser centered either at 792 nm or 812 nmwas constructed as shown in FIG. 1. The linear cavity layout 10,consisting of a 5-meter radius of curvature (ROC) high reflective rearmirror, 11 and a 10-cm radius of curvature output coupler, 12. Anuncoated 5 mm×5 mm×13 mm long 2% Nd:YCOB crystal, 13, with the Z axisalong the laser axis, was placed next to the high reflector, 11. Thepump laser polarization was rotated for maximum absorption (along theZ-axis) in the crystal and focused into the crystal with an 8.8-cmplano/convex lens, 14, through the rear mirror which was approximately82% transparent at 792 nm and 812 nm. The tunable Ti:sapphire pumpingsource 15, passed through the Faraday isolator 16, and traveled throughthe half-wave plate, 17. The half-wave plate can be used to rotate thepolarization of the pump radiation.

[0032] The required optical components for the solid state laser deviceof this invention are the active gain medium and a pumping source.Equipment items 16 and 17 are optional. The high reflector, 11 ispreferably flat and does not need the 5% radius of curvature (ROC).Focusing of the laser diode can be with any optical element, such as alens.

[0033] The experimental cavity was used to generate fundamental andself-frequency doubled laser wavelengths for both 2% Nd:YCOB and 5%Nd:YCOB. It is theoretically possible for Nd-doping to be in a range upto 50 weight %; however, it is most preferred that doping be in a rangefrom 2 to 10 weight % of the YCOB crystal. The inventors have discoveredthat Nd-doping concentration of approximately 10 weight % is mostefficient; above 10% quenching and degradation of the crystal starts.Preferably, the YCOB crystal can be anti-reflective coated. The outputpower at both the fundamental and self-frequency doubled laserwavelengths were measured for 0, 1% and 2% output coupling.

[0034]FIG. 2 shows the fundamental output power for 2% output couplingversus absorbed pump power in a 2% Nd:YCOB laser. The minimum pumpthreshold for lasing at 530 nm was determined to be 97 mW for the lowesttransmission output coupler. Slope efficiencies of 44% with fundamentaloutput powers exceeding 400 mW for 1 W of absorbed pump power wasobserved for 2% output coupling. Green self-frequency doubled outputpowers of over 0.7 mW were measured for 1 W of absorbed pump power inthis experimental laser system.

[0035] In another embodiment, efficient self-frequency doubling wasdemonstrated utilizing a 3 mm×3 mm×5 mm rotated Z-plate of 5% Nd:YCOB.Utilizing the cavity design identical to the linear cavity in FIG. 1,various pumping means were employed. The measurements shown in FIG. 3confirm the potential efficiency of a 5% Nd:YCOB laser system whenpumped by Ti:Sapphire radiation 3 a) and diode laser 3 b).

[0036] To maximize the SFD output, the output coupler was highlyreflecting at 1060 nm (R>99.9%) and highly transmitting (T>94%) at 530nm. The SFD output was optimized by adjusting the angle and hence phasematching of the crystal, by varying the mode size in the crystal, andchanging the cavity length. The SFD power as a function of absorbedTi:Sapphire pump power for a laser having an active medium of 5% Nd:YCOBis shown in FIG. 3a. Nearly 60 mW of 530 nm laser light was obtainedwith 900 mW of pump power absorbed in the crystal. The laser thresholdfor SFD output was only 23 mW of power absorbed in the crystal. Noadditional elements were inserted into the cavity to narrow thelinewidth of the laser.

[0037] Utilizing the same configuration as above, but with singlediode-pumping, 62 mW of 530 nm SFD light was generated for pump power upto 860 mW absorbed power. See FIG. 3b.

[0038] Properties, orientation and structure of the host crystal wereexamined. YCOB has a monoclinic crystal structure belonging to the spacegroup Cm (point group m).

[0039] The optical indicatrix axes (X, Y and Z) are defined relative tothe crystallographic axes (a, b and c) and planes as shown in FIG. 4. byadopting the traditional refractive index convention n_(x)<n_(y)<n_(z).The b and Y axes are colinear but opposite in direction, which isdenoted by the cross and dot signs. The crystal was cut with polishedfaces aligned at an angle of approximately 34° to the X-axis as shown inFIG. 5. The crystal surfaces were coated with an anti-reflective coatingthat had less than 1% reflection at 1060, 530 and 812 nm. The crystalabsorbed approximately 80% of the pump light. Measurements of thepolarized absorption and emission spectra of 5% Nd:YCOB for lightpolarized parallel to the X, Y, and Z axes were taken. The results areshown in FIGS. 6 and 7; confirming that the strongest absorption andemission of light occurs for light polarized parallel to the Z-axis.Referring to FIG. 6, the several strong absorption peaks in the vicinityof 800 nm make this material attractive for laser diode pumping. It isagain noted that with the addition of diode pumping, the Nd:YCOB laserprovides a type of compact, high-power visible green laser light source.

[0040] From the foregoing experiments it was observed that diode pumpedself-frequency doubling in a Nd:YCOB laser system can be demonstratedwith a dichroic mirror coated directly on the face of one of thepolished crystal surfaces. SFD laser emission has been observed withmodest pump powers from a low brightness laser diode. It is shown thatthe new material, Nd:YCOB, is a promising laser crystal for developmentof the next generation of compact, diode-pumped, solid-state, visiblelaser systems.

[0041] While the invention has been described, disclosed, illustratedand shown in various terms of certain embodiments or modifications whichit is presumed in practice, the scope of the invention is not intendedto be, nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

We claim:
 1. A solid-state laser device wherein the single active gainelement is trivalent neodymium-doped yttrium calcium oxyborate crystal[Nd³⁺:YCa₄O(BO₃)₃].
 2. A solid-state laser device of claim 1 , whereinthe host crystal is YCOB.
 3. A solid-state laser device of claim 2 ,wherein Nd-doping is in a range from approximately 2 weight % to about 4weight % of the YCOB crystal.
 4. A solid-state laser device of claim 2 ,wherein Nd-doping is approximately 5 weight % of the YCOB crystal.
 5. Asolid-state laser device of claim 2 , wherein Nd-doping is in a rangefrom approximately 6 weight % to about 20 weight % of the YCOB crystal.6. A solid-state laser device of claim 5 , wherein Nd-doping isapproximately 10 weight % of the YCOB crystal.
 7. A solid-state laserdevice with optical components comprising: a) one single elementcombining the active gain medium and the frequency doubler; and b) apumping source for optically pumping said single element of said deviceto generate a laser light output of approximately 400 mW atapproximately 1060 nm wavelength and a self-frequency doubled output ofapproximately 60 mW at approximately 530 nm wavelength, said pumpingsource being selected from one of a coherent pumping source and anincoherent pumping source.
 8. The laser device of claim 7 , wherein saidsingle active gain element is a crystal of Nd_(x)YCa₄O(BO₃)₃ where(x=0.02−0.10) and serves as a self-frequency doubling crystal capable ofperforming a frequency conversion process within the crystal.
 9. Thelaser device of claim 8 , wherein the Nd:YCOB gain medium isantireflective coated at approximately 1060 nm, approximately 520 nm andapproximately 812 nm.
 10. The laser device of claim 7 , 8 or 9, whereinsaid incoherent pumping source is selected from the group consisting ofa straight-shaped lamp, a spiral-shaped lamp, and an annuloid lamp. 11.The laser device of claim 10 , wherein said incoherent pumping source ispulsed.
 12. The laser device of claim 10 , wherein said incoherentpumping source is continuous.
 13. The laser device of claim 10 , 11 , or12, wherein said incoherent pumping source is selected from the groupconsisting of a xenon lamp, a krypton lamp, and optical spectrum matchedlaser diodes.
 14. The laser device of claim 7 , wherein said coherentpumping source is selected from the group consisting of a semi-conductordiode laser and an array of diode lasers.
 15. The laser device of claim14 , wherein said coherent pumping source is pulsed.
 16. The laserdevice of claim 14 , wherein said coherent pumping source is continuous.17. The laser device of claim 16 , wherein said coherent pumping sourcecomprises Titanium:Sapphire radiation.
 18. The laser device of claim 17, wherein the optical radiation from said coherent pumping source istuned to a wavelength that provides energy to said Nd:YCOB crystal. 19.The laser device of claim 17 , wherein the optical radiation from saidcoherent pumping source is tuned to a wavelength from approximately 760nm to approximately 800 nm.
 20. The laser device of claim 17 , whereinthe optical radiation from said coherent pumping source is tuned to awavelength of approximately 792 nm.
 21. The laser device of claim 17 ,wherein the optical radiation from said coherent pumping source is tunedto a wavelength between approximately 800 nm to approximately 805 nm.22. The laser device of claim 17 , wherein the optical radiation forsaid coherent pumping source is tuned to a wavelength betweenapproximately 805 nm to approximately 808 nm.
 23. The laser device ofclaim 17 , wherein the optical radiation for said coherent pumpingsource is tuned to a wavelength between approximately 808 nm toapproximately 815 nm.
 24. The laser device of claim 17 , wherein theoptical radiation for said coherent pumping source is tuned to awavelength of approximately 812 nm.
 25. The laser device of claim 17 ,wherein the optical radiation for said coherent pumping source is tunedto a wavelength between approximately 815 nm to approximately 840 nm.26. A method for producing a fundamental beam and self-frequencydoubling said fundamental beam to produce green laser light, comprisingthe steps of: (a) emitting optical radiation from a pump source selectedfrom one of a coherent pumping source and an incoherent pumping source;(b) pumping an active gain medium in a laser cavity with the opticalradiation of step (a), wherein the gain medium consists of trivalentneodymium-doped yttrium calcium oxyborate crystal, NdxYCa₄O(BO₃)₃ where(x=0.02−0.10); and (c) generating a fundamental beam that isself-frequency doubled to produce green laser light.
 27. The method ofclaim 26 , wherein said oxyborate crystal of step (b) is anti-reflectivecoated at approximately 1060 nm, approximately 530 nm and approximately812 nm.
 28. The method of claim 26 , wherein said green laser light ofstep (c) has a wavelength of approximately 530 nm.
 29. The method ofclaim 26 , wherein green laser light is produced in a processcomprising: a) emitting optical radiation from a coherent pumping sourcebeing tuned to a wavelength that provides energy to said Nd:YCOBcrystal; b) pumping an active gain medium in laser cavity with opticalradiation of step (a); c) producing a fundamental beam of approximately1060 nm; and d) self-frequency doubling the fundamental beam of step (c)to produce green laser light at a wavelength of approximately 530 nm.30. The method of claim 26 , wherein the optical radiation from saidcoherent pumping source is tuned to a wavelength from approximately 760nm to approximately 800 nm.
 31. The method of claim 26 , wherein theoptical radiation for said coherent pumping source is tuned to awavelength of approximately 792 nm.
 32. The method of claim 26 , whereinthe optical radiation for said coherent pumping source is tuned to awavelength between approximately 800 nm to approximately 805 nm.
 33. Themethod of claim 26 , wherein the optical radiation for said coherentpumping source is tuned to a wavelength between approximately 805 nm toapproximately 808 nm.
 34. The method of claim 26 , wherein the opticalradiation for said coherent pumping source is tuned to a wavelengthbetween approximately 808 nm to approximately 815 nm.
 35. The method ofclaim 26 , wherein the optical radiation for said coherent pumpingsource is tuned to a wavelength of approximately 812 nm.
 36. The methodof claim 26 , wherein the optical radiation for said coherent pumpingsource is tuned to a wavelength between approximately 815 nm toapproximately 840 nm.